I agree with DWayne.
As you will see in other posts that also talk about the half-axel (including one I posted a couple of months back also quite frustrated), the recommendations are always the following (and this is how I learned):
(supposing flying from right to left, so right wing top)
1. Give short push on right hand (top wing)
2. Immediately pop back right hand (top wing)
3. In parallel
with point 2) give quite some slack on left hand (bottom wing).
You will see that (as many have said) the slack on the bottom wing is essential
At this point you will hear a characteristic pop sound of the kite and it will flip with the also characteristic half axel shape.
Now, one thing that I have never been told and that I think is VERY useful is that, once you see the kite has started to do the half axel, you must pick up your lines (gently pull back both of them) to make the kite exit cleanly to the right; i.e. 180 degrees from its initial trajectory.
I wonder why no one has ever stressed this, but when I didn't do this, the kite lost its clean trick sequence and either fell out of control or rotated upwards exiting the trick in a very ugly way.
I discovered that if you start pulling gently approximately half way the trick, the kite then finishes the trick nicely.
So, summing up:
* VERY IMPORTANT, points 1, 2 & 3 --> 2 & 3 in parallel
* to end trick pull both lines gently back
One last comment. I started to get nice half axels when the kite flew to the left because I'm right handed.
When I started to try doing it on the other side, things went terrible again!
So don't get frustrated or disappointed. For right handed pilots, doing things with the left asks for some training to make your right hand to stop wanting to be the protagonist...
I'm still in this process. I can sometimes do it, but my right hand keeps getting jealous...
Hope this helps!
BTW, I'm not worrying too much at this point about doing the trick with a nice position of arms and hands. I look very much like a boxer when doing the half axel 1, 2 & 3 steps. So I guess you also have to find your style first